Test circuit and scan tested logic device with isolated data lines during testing

ABSTRACT

A test circuit for a logic device having ports. The test circuit includes a serial scan path for serially transferring externally generated test vectors from a serial test input to a serial test output. A storing circuit stores a data bit and has a node at which the data bit is stored. A first interface circuit interfaces the node with a first one of the ports for synchronous transfer of data from the logic device to the node. A second interface circuit interfaces the node with the serial scan path to tranbsfer data from the serial scan path to the node. A coupling circuit connects the storing circuit to a second of the ports to transfer a logic level responsive to the data bit to the logic device during test. Also the coupling circuit temporarily couples the data bit from the node to the serial scan path also during test. A third interface circuit is provided for an asynchronous input of data from the logic device to the coupling circuit except during test wherein the asynchronous input is isolated from the coupling circuit. A control circuit controls the third interface and the coupling circuit during test in response to an external test enable signal. In this way, the second interface is operable during test to store data in the storing circuit for input to the logic device, and the first interface transfers test results from the logic device to the storing circuit for extraction through the serial scan path.

This is a continuation of application Ser. No. 07/078,392 filed July 27, 1987, now abandoned, which is a division of application Ser. No. 06/790,598 filed Oct. 23, 1985, now U.S. Pat. No. 4,698,588.

TECHNICAL FIELD OF THE INVENTION

The present invention pertains in general to testing of very large scale integrated circuits, and more particularly, to utilization of scan testing techniques in defined test boundaries within a partitioned system.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to coassigned U.S. patent application Ser. No. 790,569, filed Oct. 23, 1985, now U.S. Pat. No. 4,710,933, Ser. No. 790,543, filed Oct. 23, 1985, now U.S. Pat. No. 4,710,931 and Ser. No. 790,541, filed Oct. 23, 1985, now U.S. Pat. No. 4,701,921.

BACKGROUND OF THE INVENTION

Several programs have been underway to increase the density of integrated circuits. One of these programs, the very high speed integrated circuit (VHSIC) program, is directed toward delivering electronics utilizing very large scale integrated circuit (VLSI) techniques to provide more advanced and smarter systems. Some of the older VLSI programs have a performance specification of five×10¹³ gate-Hz/cm² functional throughput rates. In order to achieve this type of performance, lithographic equipment that draws ultrafine lines must be built, and the requisite packages created. These chips will have features as small as 0.5 micrometers which can result in a chip having between 20,000 to 40,000 gates and 100,000 to 400,000 transistors on chips ranging as large as 0.25 square inches. They will be mounted on carriers having between 150 to 400 pins.

One of the largest challenges to developing high density VLSI devices is testing. Because of the complex circuit functions and/or large quantities of circuit functions that may be present in a VLSI circuit, or in a system that employs VLSI circuits, testing methods have themselves become very sophisticated and complex. In addition, complex test methods generally require corresponding complex equipment, including software, and become increasingly expensive and burdensome to carry out. However, in order to ensure a relatively high degree of reliability for a given chip design, it is necessary to design for testing to ensure that a high percentage of the faults are detectable. Present goals for fault detections are 98% or greater.

One of the best known test methods for testing large scale integrated circuits is level sensitive scan design (LSSD) which is well described in the literature. See U.S. Pat. No. 4,519,078, issued to Donald Komonytsky and assigned to Storage Technology Corporation for a general description of LSSD. See also U.S. Pat. No. 4,428,060, issued to A. Blum and assigned to International Business Machines, and E. J. McCluskey, "A Survey of Design for Testability Scan Techniques", VLSI Design, Dec. 1984, pp. 38-61, for a comprehensive list of patents and publications for the testing of electronic structures. Basically, LSSD utilizes a plurality of controllability/observability points internal to an LSI circuit. Controllability is provided by allowing data to be shifted into these points which are comprised of shift register latches (SRL's) in a serial manner. A test is then performed and then the data stored in the SRL's and shifted back out for observation thereof. Therefore, control/observation of an LSI circuit no longer depends on the number of pins in the package. Furthermore, because the latches themselves are part of the internal circuit, they can be utilized to break feedback paths in a sequential circuit, enabling the test for the combinational circuits between SRL's to be generated automatically.

In a typical scan design, the shift registers are located at specific points required for the design function but are connected together in a scan chain for testing purposes. The scan chain allows for realization of any test state in the registers for test application. A test pattern is then generated on a computer. The generated test pattern is then shifted into the SRL's, test vectors (selected words or groups of digital data) applied to the primary inputs or pins of the chip, the system clocks applied to perform the test, the primary output pins compared to expected vector outputs and data scanned out of the SRL's to compare it to known good test vectors. In performing this test, numerous series of test vectors are usually required for shifting into the SRL's, applying the test vectors and then shifting the results back out. This becomes somewhat time consuming when applying these tests on a conventional tester.

In conventional scan designs, the SRL's are connected in a continuous string, as illustrated in U.S. Pat. No. 4,519,078, or implemented in a parallel manner, as illustrated in U.S. Pat. No. 4,503,537, issued to William McAnney and assigned to International Business Machines Corporation. In the sequential design, a predetermined amount of time is required to test a given chip. The amount of time required to shift data into the SRL's to perform the test and shift the data out is determined by the number of SRL's in the serial chain. If only one function of the chip were to be tested, data would have to be shifted through all of the shift registers in the chain in order to test this function. For example, if a chip had multiple functions and one function required loading and unloading of the shift registers four times as compared to another function which required loading and unloading of the SRL's six times to perform the appropriate test, the shift registers for the first function would have to be loaded and unloaded an additional two times to perform the final two passes for the test pattern of the second function. The time required to shift through the registers in the first function for these additional two passes is wasted time. In addition to the additional time, additional logic is required to connect the registers together. This is true even if the registers can be more easily loaded and unloaded using the normal logic flow. This extra logic can become a primary disadvantage, especially if the registers are connected to buses.

In view of the above disadvantages, there exists a need for a test method which utilizes scan techniques that reduce the time required to scan data into the control/observation points and also reduce the time for generation of the test patterns.

SUMMARY OF THE INVENTION

The present invention disclosed and claimed herein comprises a shift register latch for facilitating testing of peripheral ports on a logic circuit. The shift register latch comprises an operating input interfaced with one of the peripheral ports and an output interfaced with another of the peripheral ports. A scan data input is provided for receiving serial data from an internal serial data link. An operating gate is provided for clocking data from the operating input to a latch node in response to receiving an operating clock signal. A scan gate is provided for clocking data from the scan data input to the latch node in response to receiving a test clock signal. An isolation circuit is disposed between the latch node and the output for operating in the isolation mode to isolate data stored at the latch node from the output and in the non-isolation mode to transfer data from the latch node to the output for input to the logic circuit. A second isolation circuit is disposed between the output of the shift register latch and the associated peripheral port to inhibit input of data to the logic device from the peripheral port during test. Control circuitry is provided to control the isolation circuits to transfer data from the latch node to the output of the shift register latch and inhibit input of data during test in response to receiving an external test enable signal. In the normal operating mode, the latch node is isolated from the output of the shift register latch and data is allowed to be input through the peripheral port to the logic circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and the advantages thereof, reference is now made to the followin taken in conjunction with the in which:

FIG. 1 illustrates a schematic block diagram of a VLSI chip in the test mode with multiple logic modules;

FIG. 2 illustrates a simplified schematic block diagram of the modules of FIG. 1 with a common scan in and a common scan out path interfaced with the modules;

FIG. 3 illustrates a simplified block diagram of the modules of FIG. 1 illustrating an alternate method to the scan paths;

FIG. 4 illustrates a functional block diagram illustrating the combined parallel and serial scan logic design;

FIG. 5 illustrates a schematic block diagram of the gating circuit for selecting the scan path in each module;

FIG. 6 illustrates a schematic diagram of the SRL's;

FIG. 7 illustrates a schematic diagram of the PRL's;

FIG. 8 illustrates a schematic block diagram of a transparent SRL;

FIG. 9 illustrates a schematic diagram of the transparent SRL;

FIG. 10 illustrates a timing diagram for the clock signals;

FIG. 11 illustrates a schematic block diagram of the logic module; and

FIG. 12 illustrates a schematic diagram of the Read/Write control for controlling the PRL's.

DETAILED DESCRIPTION OF THE INVENTION Test Partitioned Logic Circuit

Referring now to FIG. 1, there is illustrated a block diagram of a test partitioned very large scale integrated (VLSI) circuit 10. The VLSI circuit 10 can be configured in an operational mode and a test mode, the configuration for the test mode being illustrated in FIG. 1. Generally, in the operational mode of a VLSI circuit, functions are normally partitioned or "modularized". These circuits can operate in any number of modes with different modules providing control functions, storage functions, etc. These modules are generally interfaced with the exterior of the circuit through device pins to provide an interface for data, addresses and control signals. Through the use of various multiplexing circuits, the pin functions can be changed and even the internal architecture of the device.

In the test mode an internal control bus 12 is interfaced with pins 14 for control functions such as clock signals, test signals and various other control signals. An internal address bus 16 is interfaced with pins 18 to allow input of addresses to the LSI circuit 10. An internal data input/output (I/O) bus 20 is interfaced with pins 24 to allow bidirectional data flow. It should be understood that the buses 12, 16, and 20 are only interfaced with the device pins 14, 18 and 24, respectively, during the test mode. In the operational mode of the VLSI circuit 10, these pins may have other functions depending upon the particular application.

The logic in the chip 10 is segmented into functional logic modules 26a-26n. The logic is a combination of both combinatorial logic and sequential logic. Each of the functional modules 26a-26n are interfaced with the control bus 12, the address bus 16 and the I/O bus 20 in the test mode. The address bus 16 allows for addressing separate ones of the modules 26a-26n and the I/O bus 20 allows for interface of parallel data therewith. The control signals for controlling all of the modules 26a-26n are provided through the control bus 12. To provide for interface between the modules in the operational mode, an operational bus 27 is provided for carrying signals between various modules. These signals are in addition to address, data and control information carried on buses 12, 16 and 20. In the configuration of FIG. 1, the external pins of the chip are internally interfaced such that the internal logic of the chip is accessible externally to provide control over the testing procedure.

Each of the modules 26a-26n is also provided with a scan data in port (SDI) and a scan data out port (SDO). The SDI port is interfaced with a common scan data in line 28 and the SDO port is interfaced with a common scan data out line 30. The scan data in line 28 is connected to an external pin for receiving serial data and the scan data out line 30 is interfaced with an external pin for outputting serial data.

Each of the modules 26a-26n provide a defined test boundary for the functional logic when in the test mode. When this test boundary is defined, the module under test is operationally isolated from the other modules by isolating the input/output of bus 27. The combinatorial logic within the defined test boundary has various internal test points associated therewith, which test points provide the necessary control/observation for testing. The internal test points are of two types, serial test points accessible through the SDI and SDO, ports and arranged in a "daisy chain" configuration or parallel test points accessible through the I/O bus 20. When test vectors are loaded into the serial test points, this is referred to as "scanning". To load test vectors into the modules, parallel test vectors are loaded through the I/0 bus 20 and then serial test vectors loaded through the SDI and SDO ports.

The modules 26a-26n are separately addressable to allow test vectors to be selectively scanned into a given module on the SDI line 28 to select ones of the test points with parallel data being input on the I/O bus 20 and also on dedicated interface pins 32a-32n. After the test vectors have been applied to the test points associated with the functional logic within the select module, the results are "captured" at the serial and parallel test points and the results are then scanned out on line 30 and output on the I/O bus 20 and also the output pins 32a-32n. With the system of FIG. 1, a stimulus can be applied in the form of a serial test pattern and a parallel test pattern which pattern can be selectively input within the defined test boundary. Each addressable such that it can be separately tested. This is important in that each separate module 26a-26n may possess a different number of test points to provide the necessary control/observation in addition to a different test pattern sequence. To complete this test pattern sequence, multiple scanning operations may be required to perform the test.

If the serial test points in all of the modules 26a-26n were connected together in a single "daisy chain" configuration, this would require shifting the serial test vectors into all of the serial test points for each scan sequence For example, if module 26a had ten serial test points in the scan path and required ten sets of scan data to facilitate a complete test and module 26b had three serial test points in the scan path and required six sets of scan data to facilitate a complete test, the daisy chain configuration would require the data to be shifted into the serial test points of module 26b an additional four times. During these additional four scans, there is no need to observe or control the serial test points in module 26b since the test for that module was completed in the first six scans. This would require the three serial test points in module 26b to be loaded and unloaded an additional four times, this time unnecessarily added onto the overall test time.

In operation, the logic circuit is first placed in the test mode and configured as illustrated in FIG. 1. This effectively isolates the modules 26a-26n from each other to allow full control thereof during testing. This prevents signals generated from other modules from being transmitted over the operational bus 27 and interfering with test results. An address is then input on the address bus 16 and one of the modules 26a-26n selected. A test clock signal is then input on the control bus along with scan data on the SDI line 28. In addition, parallel data is also input on the primary input of the selected module and then the test performed under the control of a clock signal on the control bus 12. After the test is performed, captured results are scanned out of the select module on the SDO line 30 and also from the I/O bus 20 and the associated ones of the output pins 32a-32n. These jresults are thenanalyzed external to the chip 10 and compared with valid test vectors and then another sequence of parallel and serial test vectors, if necessary, input to the select module and applied to the associated functional logic. After testing of the select module, another module is selected and the testing continued.

In addition to providing a defined test boundary for a set of functional logic with a predetermined number of serial and/or parallel points, the use of modularized functional logic groups increases the versatility and speed of initial design. Each module 26a-26n possesses a logic. Once a test pattern is generated and debugged for a given group of functional logic within a defined test boundary, the test pattern itself is then "defined." If the logic in a given one of the modules 26a-26n is altered, or if an additional module is added, it is not necessary to alter the test patterns for the remaining modules. With the use of modularized groups of functional logic with associated test patterns, this programming time is reduced and only the portion of the test pattern for the altered module or the added module need be changed. This will be described in more detail hereinbelow.

In addition to reducing the amount of time required to scan data in and results out, the modularized defined test boundaries allow the number of gates for each test pattern to be reduced. This facilitates both fault grading and test generation. For example, if a circuit was comprised of 10,000 gates and it were divided into ten functional groups of 1000 gates each, the fault grading for each module would be reduced by a factor of approximately one hundred rather than ten. This is due to the fact that the time required for fault grading and test generation is proportional to the square of the number of gates in the circuit. Therefore, by modularizing or partitioning the circuit, fault grading can be significantly reduced.

Modularized Scan Path

Referring now to FIG. 2, there is illustrated a schematic diagram of two of the functional modules 26a and 26b illustrating, the serial scan path therein. Each of the test points has a serial shift register (SRL) disposed thereat to allow input of data thereto for application to particular functional logic (not shown) and for the capture of test results. The module 26a by way of illustration has four SRL's 34, 36, 38 and 40 arranged in a serial path, and the module 26b has three SRL's 42, 44 and 46 arranged in series. The SRL's in the modules 26a-and 26b have a predetermined relationship with functional logic (not shown) such that they constitute test points. It should be understood that any number of SRL's can be included in a particular one of the logic modules 26a-26n with the number of SRL's being defined by the particular function and the number of test points necessary to facilitate a complete test and provide the requisite control/observation.

SRL 34 in module 26a is the first SRL in the chain and has the input thereof connected to the SDI line 28 through a multiplex gate circuit 48. The SRL 40 constitutes the last SRL in the chain of SRL's and is connected to the SDO line 30 through a multiplex gate circuit 50. The gate circuits 48 and 50 are controlled by an address decode/select circuit 52 which receives an address from the address bus 16 and control signals from the control bus 12. The address decode/select circuit 52 is responsive to a select address or address select signal from the address bus 16 and also to a clock signal from the control bus 12 to interface the SRL's 34-40 with the SDI line 28 and the SDO line 30.

The module 26b is configured similar to the module 26a with an input multiplex gate 54 interfacing the first SRL 42 with the SDI line 28 and a multiplex gate 56 for interfacing the last SRL in the string 46 with the SDO line 30. An address decode/select circuit 58, similar to the decode/select circuit 52, controls the operation of the gates 54 and 56.

In operation, an address is placed onto the bus 16 to select one of the modules 26a or 26b. In one embodiment of the present invention, the address bus 16 is comprised of a plurality of module select lines, with one line connected to each of the modules. In another embodiment of the present invention, the address bus 16 inputs a digital word to each of the modules, each of the modules responsive to only one value for the digital word. Therefore, each of the modules 26a-26n can be selectively multiplexed to the SDI line 28 and SDO line 30.

After a module is selected, a predetermined number of serial data bits is input on the SDI line and the SRL's internal to the respective one of the modules 26a and 26b clocked with an externally generated test clock to shift data through the string. This is referred to as "scanning". For example, multiple clock cycles will

be required to scan the serial data into all four of the serial registers 34-40. After scanning in the data, the module 26a is then controlled to apply the test vectors and capture the results in the SRL's 34-40 for later observation thereof. This data is then scanned out on the SDO line 30. Depending upon the complexity of the functional logic interfaced with the SRL's 34-40, the SRL's 34-40 may require test vectors to be scanned in and out a number of times before a complete fault test has been performed.

To perform one scan of the module 26b, it is only necessary to cycle the test clock on the control bus 12 three times to scan the data in and three times to scan data out. This sequence is repeated a predetermined number of times for the module 26b, depending upon the particular type of functional logic interfaced with the SRL's 42-46 and the test pattern that was generated for the module.

By breaking the scan path as illustrated in FIG. 2, it is possible to isolate a given group of combinatorial logic blocks with associated SRL's within the defined test boundaries of the associated module. This isolation allows each module to possess a predetermined test pattern. Once the test pattern is generated, it is not necessary to regenerate the test pattern when another module is added or one of the existing modules is altered. In addition, only the SRL's associated with a particular module must be loaded for a given sequence of test scans for that module. This allows the functional logic in one of the modules to have data scanned in and out a predetermined number of times without requiring the data to also be scanned into another module which may be less complex, thus requiring fewer scan cycles of its SRL's. This deceases the amount of time to perform the overall test.

Although the modules 26a and 26b of FIG. 2 were illustrated with gates on both the SOI port and the SDO port, it is only necessary to provide the gates 50 and 56, on the SDO- ports. This would allow data to be scanned into all of the modules with the test being run on all the modules. However, the test vectors would only be valid for the select module and observation of the captured results in the select module is all that is required. Therefore, output of results from only the select module is all that is required. Test vectors scanned into unselected modules can be disregarded.

Referring now to FIG. 3, there is illustrated an alternate embodiment of the modularized test circuit including the scan path, wherein like numerals refer to like parts in the various figures. The module 26a has the SDI input thereto connected to an SCI line 28'. The SDI input of the next adjacent module, module 26b, is connected to the SDO output of the module 264 through a serial data line 60. In a similar manner, the SDO output of module 26b is connected to the next adjacent module through a serial data line 62. The last module in the string is connected to the SDO line 30 through a serial data line 64. Each of the modules has the SCI input and the SDO output connected together with a shunt gate 66. The gate 66 has the control inputs thereof connected to the outputs of the respective address decode/select circuits 52 or 58 through inverters 68. Therefore, when not selected the gates 66 connect the SDI input to the SDO output. Only the gate 66 associated with the select one of the modules 26a-26n is opened such that associated SRL's are in series with the scan data. Therefore, the serial data must pass through each of the modules 26a-26n, but is only scanned into the SRL's of the selected module.

Parallel/Serial Scan Design

Referring now to FIG. 4, there is illustrated a schematic block diagram of one of the functional logic modules 26a-26n, referred to generally as module "26". The module 26 illustrated in FIG. 4 utilizes a combination of parallel register latches (PRL's) and SRL's. The data on the I/0 bus 20 is interfaced with an internal data bus 70 for carrying parallel data to various locations in module 26. The internal data bus 70 is interfaced with PRL's 72, 74, 78 and 80. Each of the

PRL's 72-80 is interfaced with a combinational logic circuit 82, 84, 88 and 90, respectively. For simplicity purposes, only combinatorial logic will be discussed. However, it should be understood that other types of logic such as sequential logic can also be utilized.

Each of the combinatorial logic circuits 82-90 is a multiple input, multiple output, logic network. It includes a number of levels or stages of combinatorial circuits which may take the form of conventional semiconductor logic circuits. Each network is responsive to any unique input combination of signals to provide a interfaced with respective combinatorial logic networks 82-90 to either input data to the combinatorial logic circuit or to receive data therefrom or to both input data to and receive data from the respective combinatorial logic circuits The PRL's 72-80 therefore constitute test points.

Each of the PRL's 72-80 are addressable to selectively input data thereto or extract data therefrom and they are also controllable to suspend the data therein in order to isolate the respective combinatorial logic circuit from the internal bus 70. Therefore, each of the PRL's can be loaded with predetermined test vectors for application to the inputs or outputs of the associated combinatorial logic. This parallel loading decreases the test time for the circuit as a whole.

The various combinatorial logic circuits 82-90 associated with the PRL's 72-80 may interface with each other during normal operation of the circuit, depending upon the particular function provided by that combinatorial logic circuit. If so, it may be necessary during fault testing to isolate the various combinatorial logic circuits in module 26 in order to further control the fault testing. This is facilitated with SRL's. An SRL 92 is associated with the combinatorial logic circuit 82, an SRL 94 is associated with the combinatorial logic circuit 88, an SRL 96 is associated with the combinatorial logic circuit 90 and an SRL 98 is associated with the combinatorial logic circuit 84. An additional combinatorial logic circuit 100 receives the output of the SRL 96 with an additional SRL 102 associated therewith. The output of the SRL 94 is illustrated as being input to the combinatorial logic circuit 84 with the output of the SRL's 92-98 and 102 being interfaced with the output pins 2. Each of the SRL's 92-98 and 102 are interconnected in a serial path with the SDI and SDO ports, as illustrated by a dotted line.

Although a particular organization of PRL's, SRL's and combinatorial logic is illustrated, it should be understood that each of the SRL's merely provide a control/observation or test point within the circuit with the output thereof being directed to other combinatorial logic, another SRL, one of the PRL's or even back to the input of the combinatorial logic in a feedback path. The number of SRL's and PRL's and the location thereof is a function of the design of the circuit for testability. However, the location of the PRL's 72-80 is additionally dependent upon the circuit in the operational mode. The PRL's 72-90 may function both as parallel registers in the operational mode and parallel registers in the test mode. Therefore, each of the combinatorial logic circuits in the module 26 having PRL's associated therewith may utilize the parallel registers in the operational mode, thus utilizing already existing registers and their associated silicon area and the I/O bus that they are interfaced with in the operational mode.

With the use of a combined PRL and SRL design, it is possible to selectively input parallel data to various combinatorial logic circuits within a defined test boundary and also provide additional test points with the SRL's. The PRL's provide a much faster method of loading test vectors from the I/O bus 20. The SRL's provide a second avenue for allowing access to imbedded test points in the various combinatorial logic circuits.

Each of the modules 26 has an address decoder 104 associated therewith that is interfaced with the address bus 16. In addition, the control bus 12 is interfaced with various control logic 106. The I/O bus 20 is interfaced with the internal bus 70 through an I/O buffer 105 which can either write data to the internal bus 70 or read data therefrom. Although not illustrated as such, both the address decoder 104 and the control logic 106 are formed of combinatorial logic. Thus, they too will have SRL's associated therewith in order to provide a complete functional test of the module 26. For simplicity purposes, these SRL's are not illustrated.

The address decoder 104 is operable to receive an external address from the address bus 16 and decode this address to determine if the associated module is being selected This is a module select signal. In addition, each of the PRL's 72-80 are addressable. In the preferred embodiment, the module select function is achieved by a dedicated module select line connected to each of the modules 26a-26n. The PRL address, however, is a binary coded word which is decoded for selection of the addressed one of the PRL's 72-80.

In loading the test pattern into the SRL's and PRL's, the PRL's are loaded first and then the SRL's. The module 26 is first selected by placing the appropriate address on the address bus 16 or selecting the appropriate module select line and then controlling the I/O buffer 105 for a read operation to interface the I/O bus 20 with the internal bus 70. This effectively connects external data to the internal data bus 70. In the preferred embodiment, this is an eight bit data word. Data is then applied to the I/O bus 20 simultaneous with the address of the desired one of the PRL's 72-80. The PRL's are arranged in register banks. For example, one PRL could be a three bit PRL whereas another PRL could be an eight bit PRL. The maximum size of the PRL addressable on the address bus 16 is equal to the maximum number of bits in the internal bus 70. The design of the test pattern determines the data or test vectors that are to be input to the particular PRL. If, for example, each of the PRL's 72-80 were an eight bit PRL, only four passes would be required to fully load thirty-two registers.

After all of the PRL's 72-80 have been written to in accordance with the test pattern, the SRL's are then loaded through the SDI port with the control logic 106 providing the scan clocks to scan the data in. Once the data is scanned in, the test clock is incremented and a test performed. During this test, test vectors can also be applied to the corresponding pins 32a-32n and results observed thereon. The test results are then captured and stored in the various PRL's and the SRL's. The I/O buffer 105 is then placed in the Read mode in addition to placing the PRL's 72-80 in the Read mode. Each of the PRL's 72-80 is then addressed and the information placed onto the internal bus 70 for output on the I/O bus 20. After the PRL's are read, the contents of the SRL's are then clocked out along the SDO port. The PRL's and SRL's are then loaded again to provide another scan cycle in accordance with the test pattern. After the predetermined number of scan cycles are complete and the results analyzed and compared to valid test vectors, the next module 26 is tested. Depending upon the complexity of each of the PRL's and SRL's than another. By utilizing the modularized concept with a defined boundary around the combinatorial logic and the associated PRL's and SRL's, it is possible to isolate testing for each module such that unnecessary loading of SRL's in another module, due to fewer scan cycles required therein, is eliminated.

Referring now to FIG. 5, there is illustrated a detailed block diagram of the module 26 of FIG. 4 illustrating the multiplexing operation for the modularized, scan path of FIG. 2, wherein like numerals refer to like parts in the various figures. In order to provide control of the testing operation, a control PRL 108 is provided, which is similar to PRL's 72-80. The control PRL 108 is selected by an address decoded by the address decoder 104 with a select line 110 output therefrom to the PRL 108. Data is interfaced through the I/O buffer 105 to the internal bus 70 for input or output from the PRL 108.

The PRL 108 has one register defined as the STEST register. The STEST register is utilized for storing the STEST bit. The STEST bit is set whenever the register is selected. When the PRL 108 is selected and the STEST bit is set, this indicates the serial test mode. In order to clock serial data in, one of the lines on the control bus is the scan clock (T3) which clocks in the serial data on the SDI port. The T3 clock is input to one input of a NAND gate 112, the other input of which is connected to the STEST bit wherein the output of the NAND gate 112 is connected through an inverter 114 to a gated scan clock line (T3') 116. The T3' clock is utilized to gate information along the serial path into each of the SRL's 92-98 in the module 26.

Each of the SRL's 92-98 have two inputs, one of which is the scan input and the other of which is the regular data input. The scan data is gated in through a scan gate transistor 118 and the regular data is scanned in through a gate transistor 120. The scan transistors 118 are clocked by the T3' signal and the operational data is clocked in with the system clock HCLK, which is also input on the control bus 12. The SRL's 92-98 also provide a scan output and an operational data output. Therefore, the SRL is either in a test mode wherein data can be scanned in or out, or in an operational mode to allow the test to be run and results captured in the SRL's and then the captured results output on a scan output. The operational output is referred to by reference numeral 122. To isolate the output of the last SRL 98 and the SRL chain, an output gate transistor 124 is provided which is also gated by T3'. With the use of the output gate transistor 124, the SDO line output from the module 26 will not be connected to the common SDO line 30 unless the STEST bit is set and the T3' signal generated. The output gate transistor 124 is similar to the gate circuits 50 and 56 of FIG. 2.

Shift Register Latch

Referring now to FIG. 6, there is illustrated a schematic diagram of an SRL. The input of the SRL is connected to the gated output of the transistors 118 and 120. The gate transistor 118, as described above, provides the gated scan input to an internal node 126 and the transistor 120 provides the gated normal operational input to the node 126. This input to the gate transistor 20 is labeled REGIN. An inverter 128 has the input thereof connected to the input node 126 and the output thereof connected to a node 130. A p-channel transistor 32 has the gate thereof connected to the output of inverter 128, the source thereof connected to V_(DD) and the drain thereof connected to the node 126. The transistor 32 provides a latching function when node 126 floats with respect to gating transistors 118 and 120. An inverter 134 has the input thereof connected to node 130 and the output thereof connected to an output node 136. A refresh transistor 138 is connected between node 136 and 128 and is gated by a clock signal Q2 which, as will be described hereinbelow, is a refresh clock.

In operation, a signal is gated onto node 126 by either the transistor 118 or the transistor 120. This signal appears in the inverted form on node 130 and then the noninverted form on node 136. After the logic state has been gated onto node 126, node 126 is allowed to float. Latch 132 maintains the input of the inverter 128 at the opposite state as that of the output. The refresh transistor 138 periodically connects the node 136 to the node 126 to refresh the input of the inverter 128. Since the inverter 128 in the preferred embodiment is implemented in CMOS technology, refresh is required.

Parallel Register Latch

Referring now to FIG. 7, there is illustrated a schematic diagram of the PRL. The internal bus 70 has the data line thereof associated with the PRL connected to a node 140 which is connected to one line of the internal bus 70. The node 140 is gated to a latch input node 142 through a gating transistor 144. The gating transistor 144 is gated by a PRL Write signal PRLWRT. The signal PRLWRT is a clocked Write signal which is comprised of the AND function of an external Write signal and a clock signal H3.

An inverter 146 has the input thereof connected to the node 142 and the output thereof connected to a node 148. A feedback p-channel transistor 150 is connected from the output to the input of the inverter 146 similar to the transistor 132 of FIG. 6. An inverter 152 has the input thereof connected to the node 148 and the output thereof connected to a node 154. A refresh transistor 156 is connected between nodes 142 and 154 and is gated by the Q2 clock. In order to write the information to the node 142, it is only necessary to gate the transistor 144.

The Read operation is facilitated by inverting and gating information stored on node 148 back to node 140. The node 148 is connected to the source of a p-channel transistor 158, the drain of which is connected to a node 160. An n-channel transistor 162 is connected between the node 160 and ground. The gates of both transistors 158 and 162 are connected to a PRL read signal PRLRD. The PRLRD signal is derived from the AND function of an H1 clock and an external READ signal. The node 160 is connected to the gate of an n-channel transistor 164 which is connected between the node 140 and ground.

In operation, data is written to the node 142 by gating the transistor 144 during the H3 clock cycle and information is read from the node 142 by gating the transistors 158 and 162 during the Hl clock cycle. When node 142 is allowed to float, the refresh transistor 156 maintains the information latched therein.

In some configurations, a register latch is operable to function as an input or output device. Such circuits as input/output buffers utilize the registers in this manner. The registers are merely reconfigured such that data can be received from one terminal, stored and then output in one direction. In another mode, data is received from the circuit on the input to the register for output from the device. Therefore, there is internal circuitry present to multiplex the input to the latch such that data can either be input to the register from the internal bus 70 or input to the register from the circuit itself or other locations within the circuit. However, during test it is necessary to isolate any feedback path which could possibly alter the contents of the register. This operation is referred to as the "SUSPEND" operation.

Referring further to FIG. 7, the multiplex operation is illustrated by a gate transistor 166 connected between a signal EXTIN and the node 142. The signal EXTIN represents any signal generated by the circuitry internal to the module or from any other location. The gate of the transistor 166 is connected to the output of an AND gate 168, one input of which is connected to the inverse of a SUSPEND signal and a signal H3EXTWRT. The signal H3EXTWRT represents the AND function between the H3 clock and an external write signal. The EXTWRT signal is a signal generated internal to the module during normal operation thereof. The SUSPEND signal is generated during test external of the module and is operable to isolate the input of all registers from all other circuitry and interconnections within the module 26 with the exception of the connection to the internal data bus 70 through gate transistor 144. Thereafter, data can be written to or read from the PRL with no possibility for interference from other signals which may be input to the PRL during normal operation of the circuit.

Transparent Shift Register Latch

Referring now to FIG. 8, there is illustrated a series of the modules 26a-26n that utilize a transparent SRL 170. Only the transparent SRL 170 in module 26a is illustrated in detail. For illustrative purposes, the transparent SRL 170 is incorporated into a conventional interrupt circuit. The interrupt circuit has an input line 172 and an output line 174. Each of the modules is connected in a daisy chain configuration such that the output line 174 is connected to the input interrupt line 172 of the next adjacent module. The output line 174 is connected to the REGIN input of the SRL 170 and the input line 172 is gated to the SDO output of the SRL 170. The SDO output is illustrated as being connected to one input of a multiple input AND gate 176 and also to one input of a two input NAND gate 178. The output of the NAND gate 178 is connected to output line 174 and the other input of the NAND gate 178 is connected to an interrupt signal.

The input REGIN of the SRL 170 is gated to an internal storage node 180 through a gate 182 controlled by the clock signal H3. The. SDI input is gated to the storage node through a gate 184 by the scan clock signal T3. The node 180 is connected to the input of a gate 186 through an inverter 188. The output of the gate 186 is connected to a node 190, which node is connected to the input line 172 through a gate 192. The gate 192 is gated by the inverse of the STEST signal. The node 190 is connected to the SDO output through inverter 194. The gate 186 is connected to the output of a NAND gate 196, one input of which is connected to the H1 clock signal and the other input of which is connected to the STEST signal.

In operation, the gate 186 isolates the output of inverter 188 from node 190 during normal operation. During test, the gate 186 connects the output of inverter 188 to node 190 and allows either serial data to be scanned in from the SDI input to allow either loading of data or unloading of data. The gate 182 allows gating of data from the output of NAND gate 178 to allow capturing of data. Therefore, in the test mode, the transparent SRL 170 is operable to shift in the test pattern and shift out captured test data. However, in the operational mode, the input REGIN is isolated from the SDO output and an additional input is connected to the SDO output. This additional input gated to the SDO output provides a way to "asynchronously" connect an input to the circuit during normal operation and disconnect it during test operation. In addition, the SRL 170 functions to service two ports, thus reducing the number of SRL's required for interface pins.

The input and output pins illustrated as being interfaced with the SRL 170 are part of an interrupt logic daisy chain circuit. The AND gate 176 is operable to output a logic high signal whenever either the INT input goes low or the SDO output of the SRL 170 goes low. If the INT input on module 26a goes low, the output of the associated NAND gate 178 goes high, raising the output line 174 to a high. This is input to the module 26b on the input line 172. During normal operation, the input line 172 is connected through the SRL 170 to the SDO line and inverted. This places a logic low on the input of the associated NAND gate 178, thus raising the output thereof to a logic high. This continues down the chain for final from the module 26n. Since the transparent SRL 170 is utilized with an interrupt circuit, it is necessary that the operation be asynchronous and not clocked. Other SRL's utilized in the system require synchronization with the H3 clock during during operation of the circuit; that is, they are not transparent during normal operation. In addition, conventional SRL's must be utilized for each test point that is to be controlled/observed. With the transparent SRL 170 of the present invention, an output pin and an input pin can be controlled and observed by a single SRL.

Referring now to FIG. 9, there is illustrated a more detailed block diagram of the transparent SRL 170 of FIG. 8, wherein like numerals refer to like parts in the various Figures. The gate 182 is comprised of a pass transistor 196 which is gated by the clock signal H3. The gate circuit 184 is comprised of a gate transistor 198, which is gated by the test clock signal T3. The gate circuit 192 is comprised of a pass transistor 200, gated by the inverse of STEST, and the gate circuit 186 is comprised of pass transistor 202, which is gated by the HlSTEST signal output from the NAND gate 196.

To maintain the signal on the input of the inverter 188, a feedback p-channel transistor 204 is provided with the gate thereof connected to the output of inverter 186 and the source thereof connected to the input. In a similar manner, a p-channel gate transistor 206 is connected and feedback across the inverter 194. The feedback transistors 204 and 206 are similar in operation to the feedback transistor 132 of FIG. 6. In addition, a refresh transistor 208 is connected between the output of inverter 194 on the SDO line and the input of the inverter 188 on the node 180. The refresh transistor 208 is gated by the Q2 clock signal.

The circuit of FIG. 9 is operable in the normal operational mode to isolate node 180 from the SDO line and in the test mode to connect node 180 to the SDO line through gate transistor 202. In the test mode, the circuit of FIG. 9 operates similar to the SRL of FIGURE 6 in that a test signal clocked on to node 180 through the gate transistor 198 appears on the SDO output. The refresh transistor 208 periodically feeds this signal level back to the input of inverter 188 to maintain the signal level thereat. Each time the gate transistor 198 is clocked by clock T3, the information is stored in the latch 170 for output therefrom. Therefore, in the test mode, a predetermined stimulus can be stored in the latch for application to a desired test point. After the serial data has been scanned in, the system is tested and the gate transistor 196 clocked to input data to the latch from the REGIN port. This allows the latch 170 to "capture" data from logic that is connected to the REGIN port. During this testing, the gate transistor 200 is open, thus isolating the EXTIN port. During normal operational mode, the EXTIN port is connected to the node 190 and to the SDO output through inverter 194. In addition, any logic input to the REGIN port is isolated from the port 190 by the gate transistor 202.

The transparent latch 170 is operable to be connected to external interface pins for testing thereof. Normally, a logic circuit which outputs information from the module 26 is normally connected to the REGIN port such that data can be connected from this logic output during test. The EXTIN port is normally connected to an input pin, which signal is received from another module in the system. Since testing of a select module does not require information from another module as this will violate the defined test boundary of each of the modules 26a-26n, this information is not available. The information is provided in the test pattern. It is necessary to isolate this port to ensure that data from one of the other modules in the system does not effect the state of any of the test points during test. In addition, when connected in normal operation, the EXTIN port is "asynchronous"; that is, data appearing on the SDO output is not a function of the H3 clock. This is especially important for such circuits as interrupt circuits and various other asynchronous operational modes of the system. The transparent SRL 170 therefor provides an SRL that services two module ports, one being an input and one being an output, with the input port being isolated during test and, in normal operation, isolating the input and output ports and allowing asynchronous input on the input port.

Referring now to FIG. 10, there is illustrated a timing diagram for the clock signals. The Hl clock is a symmetrical clock which has the leading edge occurring at t=0. The H2 clock is delayed from the Hl clock by one quarter of a cycle and is a symmetrical clock signal. The H3 clock is delayed from the H1 clock by one-half cycle and is also a symmetrical clock. The Q2 clock is delayed by one-quarter cycle from the H1 clock and is asymmetrical and being present for only one-quarter of a cycle. The H1 H2 and H3 clocks are generated external to the module and the Q2 clock signal is generated internal to the module in the preferred embodiment.

Referring now to FIG. 11, there is illustrated a detailed schematic block diagram of each of the modules 26a-26n. A four bit address ADO-AD3 is input on address lines 210 to a predecode/select circuit 212. In addition, a module select signal is input thereto on a line 214. In the preferred embodiment, each of the modules 26a-26n have a separate dedicated module select line interfaced therewith. An address decoder at a remote location with respect to the modules is utilized to decode an address and provide the module select line. However, it should be understood that a binary address could be input to the module and a binary decoder utilized to decode the address.

The module select line 214 is buffered in the predecode/select circuit 212 and output on a line 216 to the control logic and timing circuit 106. The address signals ADO and ADl are predecoded with a two-to-four decoder and output on a first predecode bus 218. The addresses AD2 and AD3 are predecoded with a two-to-four decoder and output on a second four line predecode bus 220. The address lines ADO-AD3 and the module select line are part of the address bus 16.

The control logic and timing circuit 106 receives control signals from the control bus 12 in the form of READ and WRITE signals, clock signals on a clock bus 222, a SUSPEND signal and the scanning clock T3. In addition, the control logic and timing circuit 106 is illustrated as having the scan input signal SDI and the scan output signal SDO interfaced therewith. This illustrates that the control logic and timing circuit 106 has combinatorial logic inherent thereto which may have test points provided therein which require testing. Therefore, the control logic and timing circuit 106 has internal SRL's therein which must be interfaced with the SRL chain.

The control PRL 108, which was described with reference to FIG. 5, is a four bit PRL and is interfaced with an embedded logic block 224 which comprised of both combinatorial logic and sequential logic. The embedded logic block 224 has associated therewith one or more SRL's in an SRL block 225 and may be made up of a number of logic blocks with multiple SRL's. For simplicity purposes, only the single block of embedded logic associated with the PRL will be illustrated with respect to FIG. 11. The SRL block 225 has an SDI input and an SDO output, the SDI input connecgted to the SDI output of the control logic block 106.

The control PRL 108 is controlled by a READ/WRITE (R/W) circuit 226 which outputs a read and a write signal to the control PRL 108. In addition, a Q2 clock signal is input thereto for refresh purposes and a SUSPEND signal (SPND) is also input thereto to provide the suspend function, if necessary. The R/W circuit 226 provides the final stage of decoding and has one input line connected to the predecode bus 218 and one select input connected to the predecode bus 220.

In addition to the control PRL 108, additional PRL's 230a-230n are interfaced with the internal bus 70.

of the PRL's 230a-230n have the outputs thereof interfaced with associated embedded logic blocks 232a-232n, which are similar to the embedded logic block 224. Associated therewith are SRL blocks 233a-233n. The PRL's 230a-230n are controlled by R/W control circuits 234a-234n, respectively. In addition, they have the SPND signal and the Q2 clock signal input thereto. It should be understood that not all PRL's operate with the suspend feature and, therefore, may not have the SPND input.

The serial test line is connected from the SDO output of the SRL block 225 to the SDI input of the SRL block 233a for interface with the internal SRL's therein. The SDO output of the SRL block 233 is connected to the next adjacent SRL block and finally to the SRL block 232n. The output of the SRL block 232n defines the end of the chain and is output on an SCO line 236.

The SDO output 236 is input to a three input NAND gate 238. The T3 clock signal is input to one input of an AND gate 240, the other input of which is connected to a node 242. Node 242 is connected to the third input of the NAND gate 238. The STEST signal is connected through a gate transistor 244 to the input of an inverter 246, the output of which is connected to a node 242. The gate transistor 242 is gated by the Hl clock. The output of the NAND gate 238 is connected through an inverter 248 to the SDO input of the control logic and timing circuit for buffering thereof and output to the SDO line.

The operation of the NAND gate 238 provides the gating function illustrated in FIG. 5 which prevents the scan output from being interfaced with the common SDO line 30 which interfaces all the modules 26a-26n. The circuit can either be interfaced with the input or with the output. However, in the preferred embodiment, it is not necessary to disconnect the SDI line that is input to each of the modules 26a-26n since it is not important what data is scanned into registers of a module not under test. It is only important what data is scanned out. Therefore, the gating circuit on the output of the SRL block 233n inhibits scan data from being output from the module unless the STEST bit is set in the control PRL 108.

To provide a test, the module select line 214 is placed at a predetermined logic state to select the module through appropriate logic in the control logic and timing circuit 106. The module select signal controls the Read and Write operation of the I/O buffer 105 and also the Read and Write operation of the various PRL's. An I/O read signal I/ORD and the I/O write signal I/OWRT are generated in response to receiving the respective Read or Write signal and the module select signal. In addition, the PRLRD and PRLWRT signal are also generated in response to receiving the read and write signals and the module select signal. However, the Read/Write control of the PRL's depend upon the address received on the lines 210.

In operation, the module select line is raised high and an appropriate PRL address applied to the lines 210. The appropriate PRL's are interfaced with the internal data bus 70 and the I/O buffer 105 placed in the write mode to pass data to the internal data bus 70 from the I/O bus 20. The appropriate data is written to the PRL and then another address selected for writing to a subsequent PRL. This is continued until all of the PRL's have the appropriate data latched thereto. During this time, the SPND signal is also activated such that all external inputs to the control PRL's 108 from other places in the circuit or from other modules are suspended and prevented from being multiplexed with the inputs, as described above with reference to FIG. 7.

After all the PRL's have been loaded, data is scanned into the SRL's according to a predetermined pattern. After the data has been scanned in, the test is run by sequencing the clocks H1-H3 and then captured results scanned out. The test results captured in the PRL's are then unloaded. If required, additional data is placed into the PRL's and SRL's and the test rerun to test other conditions.

Referring now to FIG. 12, there is illustrated a schematic block diagram of the R/W control circuit 226, which is similar to the I/O control circuits 234a-234n, and the PRL 108. Each of the registers in the PRL bank 108 have a read input, a write input, a Q2 input and a SPND input. The Q2 input is connected to a Q2 line 250, the write input is connected to a Write line 252, the read input is connected to a Read line 254 and the SPND input is connected to a Suspend line 256.

The R/W control 226 has the two select inputs from the predecode buses 218 and 220 connected to the inputs of a NOR gate 260. The output of the NOR gate 260 is connected to one input of a NAND gate 262 and one input of a NAND gate 264. The other input of the NAND gate 262 is connected to V_(DD) and the other input of the NAND gate 264 is connected to the PRLRD signal output by the control logic and timing circuit 106. The output of the NAND gate 264 is connected to the Read line 254 and comprises the PRL read control signal. The output of the NAND gate 262 is input to one input of an AND gate 266, the other input of which is connected to an RST signal. The RST signal is a system reset which sets the PRL contents to active low when present. The output of AND gate 266 is connected to one input of a NOR gate 268, the other input of which is connected to the PRLWRT signal. The output of the NOR gate comprises the write control signal for the PRL 108 and is connected to the Write line 252.

In operation, the NOR gate 260 provides the final level of decoding with the generation of either the PRLWRT signal or the PRLRD signal controlling the write or read operation, respectively. The PRLWRT signal and the PRLRD signal are generated in accordance with the clock H1-H3. For example, the PRLWRT signal requires the presence of a Write signal, the H3 clock and the module select signal. The PRLRD signal requires the presence of the module select signal, the H1 clock and the READ signal. In the preferred embodiment, only a read signal is input to the module with the inverse thereof being WRITE such that only one input is required for two different states.

In summary, there has been provided a scan test system which utilizes partitioned modules which enclose a plurality of SRL's and/or PRL's therein within a defined boundary. Each of the modules, from a test standpoint, is independent from the other modules. Therefore, a separate test pattern can be generated for each module and each module can be tested separate from each other module, thus reducing both the time to complete a total test of the system and also the time for generating the test pattern.

Although the preferred embodiment has been described in detail, it should be understood that various changes, substitutions and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended claims. 

What is claimed is:
 1. A test circuti for a logic device having ports, the test circuti comprising:a serial scan path for serially trnasferring externally generated test vectors from a serial test input to a serial test output; a storing circuit for storing a data bit and having a node at which the data bit is stored; first interface means for interfacing said node with a first one of the ports for synchronous transfer of data from the logic device to said node; second interface means for interfacing said node with said serial scan path to transfer data from said serial scan path to said node; coupling means for connecting said storing circuit to a second of the ports to transfer a logic level responsive to the data bit to the logic device during test, said coupling means temporarily coupling the data bit fromwid node to said serial scan path also during test; third interface means for an asynchronous input of data fromthe ogic dvice to said coupling measn except during test whereinthe asynchronous input is isolated from said coupling means; and control measn for controlling said third interface maesn and said coupling means during test in response to an externaltest enable signal; said second interface measn operable during test to store data insid storing circuit forinput to the logic device and said first interface means operable to transfer test results from the logic device to said storing circuit for extraction through said serial scan path.
 2. The test circuit of claim 1 wherein sid storingn circuit comprises a latch and said first and second interface means comprise first and second gates, respectively, for gating data to said latch.
 3. The test circuit of claim 1 wherein said third interface means comprises a gate connected to said coupling means.
 4. The logic circuit of claim 1 wherein said coupling means includes a connection line between said storing circuit and the second port and further inculdes a gate and a latch, the latch having an output to sid serial scan path, thegate connected between said storing circuit and siad latch wherein said gate is closed during test.
 5. A scan tested logic device, comprising:a plurality of logic modules each including logic circuitry and data lines to perform a combined logic function in an opertional mode; a plurality of embedded shift register latches disposed at select locations within said modules for receiving test vectors in a test mode through a test input, applying the received test vectors to the logic circuitry through an output and storing results of the test, said shift register latches in the operational mode connected to said logic circuitry through an operating input and the output to allow said modules to perform said combined logic functions; a serial data link for serially connecting said shift register latches inseries inthe test mode for serially loading test vectors therein and serially unloading results therefrom; test means for isolating select ones of the data lines in the test mode to inhibit data being input fromthe data lines to the logic circuitry of each module during test; and control means for controlling said shift register latches and said test means to function in the test mode.
 6. The logic device of claim 5 wherein said shift register latches are synchronously scan clocked in the test mode and synchronously operationally clocked in the operational mode, said test means allowing asynchronous operation of the associated ones of said data lines in the operational mode.
 7. The logic device of claim 5 wherein at least one saif shift register latch has circuitry with an operating input, a test input and an output, the operating input connected to a first one of sidselect data lines and the test input threof connected inseries with said serialdata link, anda first gate interposed between parts of the circuitry of said shift register latch and operable in the test mode to connect the parts of the circuitry of said shift register latch and in the operational mode to isolate theparts of the circuitry of said shift register latch; and wherein said test means includes a second gate disposed between said second data line and sdi first gate and operable inthe tst mode to isolate said second dat line from said shift register latch and in the operational mode to connect said second data line to part of said shift register latch.
 8. A semiconductor device comprising:a logic circuitry to be tested; serial register latches connected inseries and externally serially accessible, each of said serial registe latches connected to points insid logic circuitry; and externally selectable shunt circuits connected across different sets of said serial register latches.
 9. A semiconductor device as claimed in claim 8 further comprising a selection circuit responsive to a selection signal to control the shunt circuits so that only one selected set at a time of the serial register latches, and not the rest, are accessible.
 10. A semiconductor device as claimed in claim 8 further comprising interface circuits interposed in the series of serial register latches and respectively connected to said shunt circuits.
 11. A semiconductor device as claimed in claim 10 further comprising a selection circuit responsive to a selection signal to control the shunt circuits and the interface circuits so that only one selected set at a time of the serialregister latches, and no the rest, are externally serially readable.
 12. A semiconductor device as claimed in claim 8 further comprising a chip carrier having pins for serial data input to and serialdata output from said serial register latches.
 13. A testable system comprising:carriersand chips mounted on siad carriers to constitute a scan test system having atotal time for test of the system wherein each of the chips includes logic circuitry to be tested; serial register latches connected in series and externally serially jaccessible, each of said serial register latches connected to a differnt point in sid logic circuitry; and externally selectable shunt circuits connected across different sets of said serial register latches, whereby selective shunting of the serial register latches causes the total time for test of the system to be reduced.
 14. A testable system as claimed in claim 13 wherein each of the chips further comprises a selection circuit responsive to a selection signal to control the shunt circuits so that only one selected set at a time of the serial register latches, and not the rest, are accessible.
 15. A tstable system as claimed in claim 13 wherein at least one of the chips further comprises interface circuits interposed in the series of serial register latches and respectively connected to said shunt circuits.
 16. A testable system as claimed in claim 15 wherein each chip with interface circuits further comprises a selection circuit responsive to a selectino signal to control the shunt ciuits and the interface circuits so that only one selected set at a time of the serial register latches are externally serially readable.
 17. A semiconductor device for use with a scan clock and an operational clock, comprising:logic circuitry to be tested; and serial register latches connected to be externally accessible, each of said serial register latches connected to a different point in siad logic circuitry and at least one of said serial register latches including a storing circuit and two gates respectively feeding said storing circuit from another serial register latch and from said logic circuitry, the two gates respectively controlled by the scan clock and the operational clock, said storing circuit having an output connected to said logic circuitry; a second circuit feeding a succeeding serial register latch, the second circuit having an input; and a gating circuit connected to the input of the second circuit and responsive to a test mode control signal to connect the storing circuit to the input of the second circuit when the test mode control signal is active adn alternatively to connect the input of the second circuit to an asynchronous operating input when the logic circuitry is not under test.
 18. A testable system comprising:carriers and chips mounted on said carriers to constitute a scan test system having a scan clock and an operational clock wherein each of the chips includes logic circuitry to be tested; and serial register latches connected to be externally accessible, each of said serial register latches connected to a different point in said logic circuitry and at least one of said serial register latches including a storing circuit and two gates respectively feeding said storing circuit from another serial register latch and from said logic circuitry, the two gates respectively controlled by the scan clock and the operational clock, said storing circuit having an output connected to said logic circuitry; a second circuit feeding a succeeding serial register latch, the second circuit having an input; and a gating circuit connected to the input of the second circuit and responsive to a test mode control signal to connect the storing circuit to the input of the second circuit when the test mode control signal is active and alternatively to connect the input of the second circuit to an asynchronous operating input when the logic circuitry is not under test. 